1. Field of the Invention
The invention relates to a double-walled heat insulating containers and somewhat more particularly to a process and apparatus for producing double-walled heat insulating containers.
2. Prior Art
Glass double-walled heat insulating containers, such as for heat-retaining flasks and/or ice-containing flasks are still partly mouth-blown. Such production procedures are very costly and inconvenient.
Processes for producing double-walled insulating containers via machines are also known. In such processes, an inner bulb and an outer bulb are blown separately from each other and stress-relieved. Open end portions of the bulbs are then cut-off separately. Further processing is determined by whether a wide-mouth or a narrow-mouth container is desired. When producing an insulating container with a wide-mouth, the lip of the inner bulb open end is flanged, the bulbs placed one inside the other and maintained in the required mutual disposition via a separate support means. The outer bulb is then constricted at its open end and fused with the open end of the inner bulb. The outer bulb being previously provided with a pump spigot to enable evacuation of the space between the bulbs.
Such machine processes are limited to production of wide-mouth insulating containers since the open end of the outer bulb can only be constricted a limited amount. The glass of such a bulb has to be brought to a high temperature, producing a low viscosity, in order to allow the constricting operation to take place. The greater the degree of constriction required, the larger must be the low viscosity range of the glass and the longer its plastic or deformable state must be maintained. With the low viscosities which are thus encountered, surface tension forces occurring in the glass cause it to contract in order to assume a smaller surface area and consequently the glass changes dimensionally. In the radial direction, it is possible to counter this change by using centrifugal force applied via rotation of the glass. On the other hand, in the longitudinal direction, there is no way of counteracting the surface tension effects so that the open end of the outer bulb is subject to shrinkage in that direction.
Additionally, the wall thicknesses inevitably vary from bulb to bulb and this in itself means that a uniform and time-constant shrinkage pattern is unattainable and the result is dimensional variations in the ultimate containers. In view of these irregularities, it is necessary to limit the amount of constriction of the outer bulb, thereby limiting very considerably the possibility of changing container shapes.
Since narrow-mouth insulating containers require the open end of the outer bulb to be left wide at the beginning of the process in order to allow the insertion of the inner bulb therein, a relatively marked subsequent constriction of the outer bulb is necessary. However, this is not possible with the process described above so that its application to the production of narrow-mouth insulating containers is completely excluded. A prior art solution to this problem has been to blow the mouth of the outer bulb to its final dimension, cut the outer bulb in half and then fuse the so-cut halves together around the inner bulb. However, both known processes variations are costly and inconvenient and result in qualitatively poor fusing.